1. Field of the Invention
The present invention relates to a transmission system for a human-powered vehicle. More particularly, the present invention relates to an automatic transmission system for a human-powered vehicle, such as a bicycle.
2. Description of the Related Art
Human-powered vehicles, such as bicycles, are usually equipped with multi-geared transmission systems that are shifted between incremental gear ratios for making changes in torque applied to the rear wheel, thus providing a vehicle operator with a plurality of possibilities for achieving a desired pedalling cadence and riding speed. The need for multi-geared transmission systems for bicycles and other human-powered vehicles arises from a variety of riding environments combined with different riding styles. For example, a racer tackles hills in a much different way than a rider out for an afternoon tour or a bicycle commuter on the way to work.
While some cyclists prefer manually shifting bicycles transmissions, other cyclists are concerned only with experiencing an enjoyable ride and do not want the burden of gear shifting decisions and manipulations. The latter type of cyclist, and those cyclists who have not learned how to shift properly, may easily cause damage to a bicycle and its transmission. More importantly, the ride experience for these types of cyclist turns out to be rough and uncomfortable.
A variety of shifting systems, some including programmable microprocessors, have been developed for addressing these problems. For example, U.S. Pat. No. 5,261,858 to Browning discloses a shifting system that does not allow shifting when a cyclist is not pedaling. The Browning system has the disadvantage that after a cyclist coasts to a quick stop, the transmission may be left in a high gear ratio. Since the system does not allow shifting when the cyclist is coasting, it is difficult for the cyclist to resume pedaling.
U.S. Pat. No. 5,213,548 to Colbert discloses a shifting device that has a limited number of inputs to the microcontroller of the device. Shifting decisions are made on wheel speed and cadence alone without the benefit of any other relevant information, when in reality, upcoming terrain has a large impact upon a cyclist's shifting decisions.
U.S. Pat. No. 5,356,348 to Bellio discloses a system that provides a constant cadence function, but requires a cyclist to choose from a preset, unchangeable range of cadence values that, once set, are not conveniently changed.
U.S. Pat. No. 5,254,044 to Anderson discloses a system that relies upon a timed actuation of multiple switches for successfully programming the system. A cyclist may become frustrated with the operation of the device and may decide to stop using it altogether.
Although prior systems are reliable and effective in many respects, they offer limited flexibility and provide operating modes having few features. What is needed is a bicycle transmission system that adapts to the style of a cyclist instead of the forcing of the cyclist to adjust to the system; that utilizes information related to the riding environment and the cyclist's conditioning in making shifting decisions; that is easier to use than existing systems; and that is more effective than previous systems for increasing a cyclist's efficiency and safety.